1. Field of the Invention
The present invention concerns a process for dimerizing olefins. In particular, the present invention concerns a process for dimerizing C4- and C5-olefins.
The present invention also concerns new fuel components as well as novel hydrocarbon compositions.
2. Description of Related Art
The octane number of the automotive fuels is increased by adding components with a high octane number, such as methyl-tert-butylether, MTBE. Alternatively, C4-alkylate or isomerates can be used. The alkylate is typically produced by alkylating isobutane and isobutene, whereby trimethylpentanes and dimethylhexanes are obtained. By dimerizing isobutene to iso-octene and hydrogenating it further to iso-octane the production of a component equal to or better than alkylate is possible.
C5-fraction has previously been used for producing ethers, such as tert-amyl methylether, TAME or tert-amyl ethylether, TAEE. Both these ethers have been used together with or instead of MTBE to increase the octane number of the automotive fuels.
The octane numbers (Research Octane Number, RON and Motor Octane Number, MON) of iso-octane are by definition 100.
The present process can also be used to dimerize linear butenes or a mixture of isobutene and linear butenes. The octane numbers of the formed products are not as high as the octane numbers of iso-octane, but also these reaction products can be used as fuel components.
In the art a process is known, in which MTBE and iso-octene are produced simultaneously (EP-A-745576). According to the publication the molar ratio of alcohol and iso-olefin has to be below the stoichiometric ratio or in the range of 0.2-0.7. If the ratio is greater than 0.7, only less than 10 wt-% of dimer is formed. The preferred lower limit depends on the composition of the feed and the alcohol (methanol or ethanol) used. It is stated in the publication that the selectivity of the dimers increases, when the molar ratio increases, but the percentage of the dimers in the product decreases. In other words, the yield of dimers can not be increased, because the amount of MTBE would increase. In addition, there is no mention in the publication of the use of other oxygen containing components for inhibiting the side-reactions.
An other process for producing both C4-oligomers and alkyl-t-butylether is known from EP-0 048 893. In the publication, a high feed ratio of alcohol and isobutene is used. In the publication a reference is made to the possibility of recycling the product in order to produce longer oligomers.
EP-publication 0 082 316 discloses a MTBE-process comprising a distillation column with a side reactor. The flow from the side reactor can be fed either to prereactors or back to the distillation column. In this case, too, the ratio of methanol and isobutene is close to stoichiometric and the purpose of the side reactor is to increase the conversion to MTBE.
It is known in the art that oxygen-containing molecules, such as methanol, MTBE, tert-butylalcohol (TBA) and water increase the dimer selectivity and thus decrease the selectivity of the trimerizing or tetramerizing reactions when dimerizing olefins in the presence of an ion-exchange resin catalyst. In that connection, we refer to what is stated in U.S. Pat. Nos. 4,375,576 and 4,100,220.
GB-application 2 325 237 discloses a process for selective dimerization of isobutene, in which primary alcohol and alkyl ether are fed to the process together with isobutene-containing hydrocarbon feed. The molar ratio of alcohol to isobutene is less than 0.2 in the feed. The molar ratio of alcohol and alkyl ether together to isobutene in the feed is more than 0.1. It is, however, stated in the publication that the best range of the latter molar ratio actually varies from between 0.2 and 0.6 to between 0.3 and 0.6 and between 0.5 and 0.7 depending on the composition of the hydrocarbon feed. Thus, the molar ratio in the feed is kept relatively small.
In prior art, no such process is known, which would allow for free selection of the product composition of the dimerizing unit and enable the production of either pure dimer or a mixture of dimer and ether in the same unit.
The objective of the present invention is to eliminate the problems of prior art and provide a novel process for dimerizing olefinic feedstocks.
The invention is based on the idea that the C4- and C5-olefins are dimerized in the presence of alcohol or another oxygenate in a reaction sequence comprising at least one distillation zone and at least one reaction zone. The reaction is carried out at conditions in which at least part of the olefins dimerize. The distillation zone is arranged after the reaction zone, and a flow comprising oxygenate, like, for example, alcohol, water or the product(s) of reaction(s) between alcohol or water and the olefin(s) present in the feed, or a mixture of any or all of these is circulated from the distillation zone back to the dimerization. The circulation flow(s) is (are) drawn from the side of at least one distillation column. The molar ratio of alcohol or other oxygenate and isobutene is adjusted to be small during the reaction, thus maintaining the rate of dimerization high.
According to another process according to the present invention, the sidedraw is directed to another reaction zone and the overhead product is circulated back to the dimerization.
The process according to the present invention can be used to produce dimerized products from feeds containing olefinic hydrocarbons selected from the group of linear butenes, isobutene and linear or branched C5-olefins. Alternatively, the feed can comprise a mixture of any or all of the olefins listed above.
According to a first preferred embodiment of the invention, the hydrocarbon feed containing isobutene or linear butenes or a mixture thereof, is contacted with an acidic catalyst together with alcohol or other oxygenate in a reaction system comprising at least one reaction zone and at least one distillation zone. The conditions in said reaction zone are such that at least a part of the isobutene is dimerized to iso-octene. The flow from said reaction zone is introduced into a distillation zone, where the main part of the dimerized reaction product is separated. A sidedraw comprising alcohol, other oxygenate or the reaction product or a mixture thereof is circulated from the distillation zone back to the dimerization. With the help of the sidedraw the conversion of isobutene and the production of dimerized product is increased.
According to the first preferred embodiment, the hydrocarbon composition produced by the process of the present invention comprises at least 85 wt-%, preferably 90 wt-% iso-octene, 10-4 wt-%, in particular 10-6 wt-% trimers of isobutene, less than 1 wt-% tetramers of isobutene, 0.02-2 wt-%, typically 0.5-1.5 wt-% MTBE and 1 wt-% or less other hydrocarbons. When the composition is hydrogenated, an iso-octane composition useful as a fuel component is obtained.
According to a second preferred embodiment of the invention the hydrocarbon feed contains olefins selected from the group of linear and branched C5-olefins, or a mixture thereof. Thus, the olefins typically present in the feed comprise linear 1-, 2- or 3-pentene, 2-methyl-1-butene, 2-methyl-2-butene and 3-methyl-1-butene.
According to the second preferred embodiment, the hydrocarbon composition produced by the process of the present invention comprises at least 65 wt-%, preferably at least 75 wt-%, C5-dimers, 5-32 wt-% , preferably 5-28.5 wt-% olefin trimers, less than 1 wt-%, preferably less than 0.5 wt-% olefin tetramers, and 0.001-2 wt-%, preferably 0.001-1 wt-% oxygenate. Oxygenate can be for example MTBE or TBA, depending on the oxygenate used in the process. When the composition is hydrogenated, a composition useful as a fuel component is obtained.
According to third preferred embodiment of the invention the hydrocarbon feed contains olefins selected from the group of isobutene, linear butene, linear and branched C5-olefins, or a mixture thereof. Thus, the olefins present in the feed possibly comprise any or every one of those described above.
According to the third preferred embodiment, the hydrocarbon composition produced by the process of the present invention comprises at least 65 wt-%, preferably at least 70 wt-% dimers or C9-olefins, 5-32 wt-%, preferably 5-28.5 wt-% trimers, less than 1 wt-%, preferably less than 0.5 wt-% tetramers, 0.001-2 wt-%, typically 0.001-1 wt-% oxygenate. When the composition is hydrogenated, a composition useful as a fuel component is obtained.
Considerable advantages are achieved by means of the present invention. When using the process of the present invention, iso-olefins can be converted to their dimers or to tertiary ether almost completely. In addition, a more dimer selective process with a smaller alcohol feed than known in the art can be achieved, thus making the production more efficient compared with previously used processes.
With the aid of the invention an isobutene processing plant, such as MTBE-unit, can be modified to a dimerization unit without high expenses. Similarly, a C5-olefin (e.g. isoamylene) processing plant, such as TAME unit can be modified to a dimerization unit. At the conditions where dimer is formed, the fraction containing ether or alcohol or a mixture thereof, is taken as a sidedraw from the distillation column and circulated back to the reaction zone. The ether or alcohol functions as an oxygen-containing component and decomposes in the reaction zone at least partly to alcohol and olefin. When all the ether is circulated back, then dimers and minor amounts of trimers and heavier hydrocarbons are produced, while if part of the ether is recovered, then alcohol is preferably added in order to maintain the conditions beneficial for dimer selectivity.
The conditions in the reaction zone can be optimized to match different production objectives. The process according to the present invention is suitable for dimerizing C4- olefins, C5-olefins or mixtures thereof. The switch from one product to another is simple, thus creating perfect flexibility to answer to the demands of the changing market.
With the aid of the recycling flow the temperature in the reactor can be slightly lowered compared to conventional etherification process. This is due to the fact that etherification is an exothermic reaction and less ether, including the undesired dimethyl ether, is formed since, according to the present invention, the methanol feed is smaller to begin with. The use of ethers as oxygenates is preferred in some cases, as the relatively high amount of alcohol in the first reaction zone easily reacts with the olefins to form the corresponding ether, and thus more heat is generated than when ether is the oxygenate originally fed to the reaction zone.
The rate of reaction can be increased by increasing the temperature in the process. This is especially preferred when TBA is used as the oxygenate.
The use of water as the oxygenate facilitates the separation, since the alcohol recovery unit is not needed. Furthermore, the amount of recycling flow decreases significantly compared with the use of primary alcohols. Further still, there will not form diethers of primary alcohols, which is a considerable advantage, since dialkyl ethers are light components for which is hard to find further use. All this is achieved with a very small amount of water.
The investment costs and the use of two separate distillation columns are much more expensive compared with a case where a sidedraw is taken from a distillation column. When only one column is used, the column has to be bigger, but savings are gained since the expensive parts, such as reboiler, condenser and instrumentation are not needed in duplicate.
Further, when considering retrofitting of plants, is much more easier to fit in only one column, possibly only modify an existing column, than try to make room enough for two columns instead.
The hydrocarbon composition obtained after hydrogenation of the reaction product of isobutene dimerization is better than the iso-octane produced conventionally by alkylation, since over 65 wt-%, typically more than 85 wt-% is 2,2,4-trimethylpentane, which has a beneficial influence on the octane number of gasoline.
The hydrocarbon compound obtained after hydrogenation of dimerized C5-fraction contains predominantly tetramethyl hexane, which has the most beneficial influence on the octane number of gasoline of all the C10 isomers.
In the conventional alkylation processes extremely acidic catalysts are used. Olefins react with acid forming red oil. Red oil is also called acid soluble oil, ASO. In alkylation processes, a liquid acidic catalyst, such as H2SO4 or HF is used. In the present invention, a solid catalyst is used and the oxygen-containing compound protects the catalyst.